Wireless communication called “millimeter wave” communication can realize higher communication speed by using a high-frequency electromagnetic wave. Examples of the main applications of millimeter-wave communication include wireless access communication for a short distance, an image transmission system, simplified wireless communication, and automobile collision prevention radars. Furthermore, at present, technology development for millimeter-wave communication, which is directed toward use promotion, such as realization of large capacity and long distance transmission, size reduction of wireless apparatuses, and reduced cost, has been performed. Here, the wavelength of a millimeter wave corresponds to 10 mm to 1 mm, and the frequency corresponds to 30 GHz to 300 GHz. For example, in wireless communication using a 60 GHz band, since channel assignment is possible in GHz units, very high-speed data communication can be performed.
A millimeter wave has a shorter wavelength and a stronger property of rectilinear propagation compared to microwaves that have become widely popular in a wireless LAN (Local Area Network) technology or the like, and can transmit a very large amount of information. On the other hand, since the attenuation of a millimeter wave as resulting from reflection is intense, for a wireless path for performing communication, a direct wave, and a wave reflected approximately one time at most are mainly used. Furthermore, since the propagation loss of a millimeter wave is large, a millimeter wave has a property such that a radio signal does not reach far places.
In order to compensate for such a travel distance problem of a millimeter wave, a method is considered in which an antenna of a transmitter/receiver is made to have directivity, a transmission beam and a reception beam thereof are directed in a direction in which a communication party is positioned, and a communication distance is extended. The directivity of a beam can be controlled by, for example, providing each of transmitters/receivers with a plurality of antennas, and by changing the transmission weight or the reception weight for each antenna. In millimeter waves, since reflected waves are hardly used, and a direct wave is important, beam shaped directivity is suitable, and a sharp beam is used for directivity. Then, after the optimum directivity of the antenna is learned, millimeter-wave wireless communication may be performed.
For example, a wireless transmission system has been proposed in which second communication means using communication of any one of electrical power line communication, optical communication, and sound wave communication transmits a signal for determining the directional direction of a transmission antenna, and the direction of the transmission antenna is determined, and thereafter, first communication means performs wireless transmission among transmitters/receivers using a radio wave of 10 GHz or higher (see, for example, Patent Document 1).
Furthermore, a method of extending a communication distance by using the directivity of an antenna has been used in IEEE 802.15.3c, which is a standard specification of wireless PAN (mmWPAN: millimeter-wave Wireless Personal Area Network) using a millimeter-wave band.
By the way, in wireless communication, it is known that a hidden terminal problem such that an area in which communication stations cannot directly communicate with one another exists occurs. Since negotiation cannot be made among hidden terminals, there is a probability that transmission operations will collide with one another. As a methodology for solving a hidden terminal problem, CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) based on an RTS/CTS handshake procedure is generally known, and IEEE 802.11 or the like has been widely used in wireless LAN systems.
In the RTS/CTS scheme, the communication station of the data transmission source transmits a transmission start request frame RTS (Request To Send), and starts the transmission of data frames in response to the reception of an acknowledgement frame CTS (Clear To Send) from the communication station of the data transmission destination.
Here, each of the control frames of RTS and CTS has a meaning of confirming the preparation situation for data transmission among transmitters/receivers and making hidden terminals in the surroundings not obstruct data transmission. A hidden terminal for the data transmission side (RTS transmission station) receives a CTS destined for another station and sets a transmission stop period (NAV: Network Allocation Vector). Consequently, it is possible for a data receiving side (CTS transmission station) to avoid a collision with a transmission frame by the associated hidden terminal, and can reliably receive the data frame. Furthermore, a hidden terminal for the data receiving side (CTS transmission station) receives an RTS destined for another station and sets a transmission stop period.
In the wireless PAN standard IEEE 802.15.3c (described above) using a millimeter-wave band, also, a collision avoidance procedure using an RTS/CTS handshake has been adopted. In this case, beamforming of a transmission/reception beam is used with regard to data frames, and control frames, such as RTS, CTS, and ACK, are transmitted as omni-directional frames (see FIG. 11).
However, in practice, peripheral stations that do not interfere with data frames are unnecessarily made to set a transmission stop period due to a difference in the range that a data frame and a control frame reach. As a result, the number of communication stations that can be communicated with at the same time in the system is reduced, and there is a concern that the throughput of the entire system is decreased.
As shown in FIG. 11, when a data receiving side (STA_B) receives an RTS destined for its own station from the data transmission side (STA_A), the data receiving side (STA_B) transmits an omni-directional CTS and causes a hidden terminal (STA_C) that an RTS does not reach but a CTS can reach to set a transmission stop period, thereby securing a period in which the data frame is received. It is assumed that the hidden terminal (STA_C) similarly performs the beamforming of the transmission/reception beam, and that the transmission/reception beam is directed in a direction different from that of the transmission/reception beam of the data frame. Despite the fact that the hidden terminal (STA_C) does not obstruct the data frame from a practical standpoint, the hidden terminal (STA_C) unnecessarily sets a transmission stop period as a result of the reception of the omni-directional CTS, with the result that the throughput is caused to decrease.